There are two forms of try statement:
try...except and
try...finally. These forms cannot be mixed (but
they can be nested in each other).

The try...except form specifies one or more
exception handlers
(the except clauses). When no exception occurs in the
try clause, no exception handler is executed. When an
exception occurs in the try suite, a search for an exception
handler is started. This search inspects the except clauses in turn until
one is found that matches the exception. An expression-less except
clause, if present, must be last; it matches any exception. For an
except clause with an expression, that expression is evaluated, and the
clause matches the exception if the resulting object is ``compatible''
with the exception. An object is compatible with an exception if it
is either the object that identifies the exception, or (for exceptions
that are classes) it is a base class of the exception, or it is a
tuple containing an item that is compatible with the exception. Note
that the object identities must match, i.e. it must be the same
object, not just an object with the same value.

If no except clause matches the exception, the search for an exception
handler continues in the surrounding code and on the invocation stack.

If the evaluation of an expression in the header of an except clause
raises an exception, the original search for a handler is canceled
and a search starts for the new exception in the surrounding code and
on the call stack (it is treated as if the entire try statement
raised the exception).

When a matching except clause is found, the exception's parameter is
assigned to the target specified in that except clause, if present,
and the except clause's suite is executed. All except clauses must
have an executable block. When the end of this block
is reached, execution continues normally after the entire try
statement. (This means that if two nested handlers exist for the same
exception, and the exception occurs in the try clause of the inner
handler, the outer handler will not handle the exception.)

Before an except clause's suite is executed, details about the
exception are assigned to three variables in the
sysmodule: sys.exc_type receives
the object identifying the exception; sys.exc_value receives
the exception's parameter; sys.exc_traceback receives a
traceback object(see section 3.2)
identifying the point in the program where the exception occurred.
These details are also available through the sys.exc_info()
function, which returns a tuple (exc_type, exc_value,
exc_traceback). Use of the corresponding variables is
deprecated in favor of this function, since their use is unsafe in a
threaded program. As of Python 1.5, the variables are restored to
their previous values (before the call) when returning from a function
that handled an exception.

The optional else clause is executed if and when control
flows off the end of the try clause.7.1 Exceptions in the else clause are not handled by the
preceding except clauses.

The try...finally form specifies a `cleanup' handler. The
try clause is executed. When no exception occurs, the
finally clause is executed. When an exception occurs in the
try clause, the exception is temporarily saved, the
finally clause is executed, and then the saved exception is
re-raised. If the finally clause raises another exception or
executes a return or break statement, the saved
exception is lost. A continue statement is illegal in the
finally clause. (The reason is a problem with the current
implementation - this restriction may be lifted in the future). The
exception information is not available to the program during execution of
the finally clause.

When a return, break or continue statement is
executed in the try suite of a try...finally
statement, the finally clause is also executed `on the way out.' A
continue statement is illegal in the finally clause.
(The reason is a problem with the current implementation -- this
restriction may be lifted in the future).

Additional information on exceptions can be found in
section 4.2, and information on using the raise
statement to generate exceptions may be found in section 6.9.